I. Field of the Invention
The present invention relates to a method of manufacturing a shadow mask for a color picture tube and, more particularly, to an etching method of a shadow mask material consisting of an iron-nickel alloy.
II. Description of the Prior Art
A shadow mask for a color picture tube is generally made of high-purity low-carbon steel such as rimmed steel or aluminum killed steel. The material is determined with regard to material feed capacity, cost, workability, mechanical strength and the like. However, such a material has a high thermal expansion coefficient (about 12.times.10.sup.-6 /.degree.C. at a temperature of 0.degree. to 100.degree. C.). In a shadow mask type color picture tube, when the relationship between positions of apertures of the shadow mask and corresponding phosphor layers is varied outside an allowable range, a so-called purity drift inevitably occurs. Particularly, thermal expansion causes a change in a required curvature of the shadow mask, thereby causing mislignment between the apertures of the shadow mask and the phosphor layers. Therefore, various means for eliminating thermal deformation of the shadow mask have been proposed.
For example, a shadow mask which uses an iron-nickel alloy having a low thermal expansion coefficient, e.g., a 36Ni-Fe invar alloy (about 2.0.times.10.sup.-6 /.degree.C. at a temperature of 0.degree. to 100.degree. C.) or a 42Ni-Fe alloy (about 5.0.times.10.sup.-6 /.degree.C. at a temperature of 0.degree. to 100.degree. C.) as a raw material has been proposed in Japanese Patent Publication No. 42-25446 and Japanese Patent Disclosure Nos. 50-58977 and 50-68650.
Recently, in a display for a personal computer, teletext, satellite broadcasting, CAPTAIN system and the like, high resolution, high contrast and good image quality are required. In order to meet these requirements, an aperture pitch of the shadow mask has become fine, and an electron beam amount emitted from an electron gun has become large. As a result, a landing reservation of the electron beam on a phosphor screen is decreased and purity drift due to thermal deformation is considerable. Therefore, an iron-nickel alloy with a low thermal expansion coefficient as mentioned above has become increasingly important. However, when an iron-nickel alloy such as invar contains rolled texture, it has a higher etching rate in a direction parallel to a rolling direction than that in a direction perpendicular thereto (direction of thickness) in comparison to a low carbon steel used as a conventional shadow mask material and containing iron as a major component. For this reason, an etching amount undesirably varies, and edges and side walls of apertures are roughened as shown in FIGS. 1A and 1B and 2, thereby forming irregular apertures. Therefore, sizes of apertures slightly vary and mask uniformity is degraded. In order to eliminate the problem of rolled texture, it is proposed that the material is subjected to a thermal treatment after cold rolling so as to adjust a size of a crystal grain. However, in such a case, an etching rate in a crystal plane is higher than that in a crystal grain boundary unlike in the case of pure iron. Then, a shadow mask material to be etched undesirably comprises a polycrystalline structure having surfaces some of which can be easily etched and others which cannot. Therefore, when this shadow mask material is etched, apertures having nonuniform shapes and slightly different sizes are formed, thus obtaining a shadow mask having a low mask uniformity. When the size of apertures of the shadow mask is large as in a conventional color picture tube, the disadvantageous effects caused by the nonuniformity of apertures are negligible, because the area of roughened edges and side walls is small with respect to the overall area of apertures. However, in a high resolution color picture tube, when a size and pitch of apertures of the shadow mask are small, the area occupying of the roughened edges which cause a slight variation of the aperture sizes is large with respect to the overall area of apertures, thus degrading white uniformity of the color picture tube.